Unlike the conventional LPG (Liquefied Petroleum Gas) engine, an LPI engine is mounted with a pump in a bombe for transferring Liquefied Petroleum (LP) gas via a fuel line and to allow the LP gas to be ejected from an injector. LP gas abruptly increases a saturated steam pressure (the increase described by a parabola) when heated, to cause a temperature thereof to increase. When the temperature in the engine room rises, pressure in the fuel line increases. When an engine operates under normal temperature conditions (under a fully heated state, not a warm-up state), and is stopped thereafter, fuel in a fuel line receives radiant heat from the engine that causes pressure in the fuel line to rise. The operating pressure in a pressure regulator becomes abnormal, causing the fuel to pass through the pressure regulator and to return to a bombe, resulting in a state of liquefied fuel and gasified (vaporized) fuel being mixed and co-existing.
The mixed state of the fuel is worse under a partial cool-down state where an engine has been turned off for 20-30 minutes, which causes the temperature in the engine room to remain high. When LP is changed from a liquefied state to a gasified state, the volume of the fuel increases approximately 250 times, so even if a small portion of gasified fuel is injected, the fuel-air mixture becomes very thin and causes problems in the smooth starting of an engine.